Subscriber's exchanges operating on the basis of time division multiplex or small spatial networks require a cheap subscriber's line circuit on a "one per line" basis. This can be realised by means of the highest possible degree of integration of the line circuits as well as by means of the highest possible packing density of these line circuits on a printed circuit board in a rack of a telephone exchange.
In order to supply the line direct current, the supply source of the prior art line circuit comprises a d.c. voltage source which usually supplies a voltage of 48 Volt or 60 Volt. The value of this line current depends on the lengths of the subscriber's line. To prevent too high a line current from flowing when very short lines are used or in the event of a short circuit of the subscriber's line, the administrations require a supply resistor of 400 to 800 Ohms to be included in series with the line. This value has been chosen so that the direct current is still of a sufficient value (approximately 20 mA) for very long subscriber's line and that the maximum direct current is limited to 400 to 120 mA.
Since a subscriber's line is a symmetrical two-wire transmission line, it must be terminated with the impedance of the subscriber's line to ensure optimum transmission of the speech signals and, for each wire of the subscriber's line, the line circuit must include an amplifier which is of such a construction that the line circuit is also of a symmetrical construction. Thus, the supply resistor in the line circuit described in the above-mentioned U.S. Pat. has been realised by means of two resistors, each of which is arranged in series with a respective wire of the subscriber's line, each resistor having half the value of the supply resistor. These resistors constitute at the same time the terminating impedance of the subscriber's line.
In order to prevent common mode signals occurring on the subscriber's line (originating, for example, from a capacitive coupling of the two wires to common mode noise sources, such as unbalanced adjacent channels or the main supply) from being converted into differential mode signals owing to resistors whose values are not equal, the administrations require an accuracy of approximately 0.1% for the said resistors. At the above-mentioned values of the resistors and supply voltages, each of these resistors could dissipate up to about 3 Watts with very short lines, which, in combination with the required accuracy of 0.1%, renders these resistors expensive. In addition, a plurality of line circuits are mounted on one p.c. board in one rack of the telephone exchange. The above-mentioned large dissipation in the resistors is the reason that the packing density on a p.c. board cannot be high or that forced cooling of a large capacity is required.
It is an object of the invention at least to mitigate these drawbacks and to provide a line circuit the dissipation of which is considerably reduced with respect to the known line circuit.